Connected: An Internet Encyclopedia
Autonomous System (AS)

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### Autonomous System (AS)

In the arcane world of exterior routing and default-less routers, Autonomous Systems (AS's) reign as the world's largest routing entities. AlterNet, CERFnet, SPRINTlink - all are Autonomous Systems. See RFC 1772 for a decent discussion of AS's and BGP's use to route among them.

In technical terms, an AS number is a 16-bit integer assigned by InterNIC and used by BGP to implement policy routing and avoid top-level routing loops. A more enlightening view is of an Autonomous System as a collection of CIDR IP address prefixes under common technical management. If you draw a network map of AS's, three distinct types can be identified:

- A Stub AS is only connected to one other AS. For routing purposes, it could be regarded as a simple extension of the other AS.

- A Transit AS has connections to more than one other AS and allows itself to be used as a conduit for traffic (transit traffic) between other AS's.

- A Multihomed AS has connections to more than one other AS, but does not allow transit traffic to pass, though its interior hosts may route traffic through multiple AS's.

It is assumed that interior routing policies and protocols have been established within each AS, enabling it to route packets internally. A Stub AS can usually have a default route to its parent. A Transit AS must use default-less routers. A Multihomed AS may use a default route to one of its neighbor AS's, but this will probably result in poor quality routing. Running BGP in a no-transit configuration is recommended.

Connected: An Internet Encyclopedia
Autonomous System (AS)